Conquering the Himalaya(s) Part 3 – Pilot jet
As I mentioned in Part 2, having filled the bike with Shell V Power and feeling much better about the bike's potential, I found that when I adjusted the idle mixture, I was unable to get a rich end to the adjustment.
Time for some carburettor theory. The amount of fuel which the engine can draw at idle is ultimately limited by the size of the pilot jet. However, there is a finer adjustment of the idle mixture provided by the idle mixture screw. Essentially, fuel is drawn through the pilot jet by the venturi effect of air passing through the carburettor and is more finely controlled by adjusting the idle mixture screw. If the pilot jet is the correct size, turning the screw inwards will restrict the fuel flow until a point is reached where the engine will falter and stall. The opposite occurs when the screw is turned outwards - more fuel is allowed to pass and eventually the mixture will become too rich at which point the engine will again falter and stop. The most correct mixture is found somewhere between the two extremes where the engine's idle speed is the most stable.
In the case of the Himalayan, turning the screw inwards to lean the mixture caused the idle to drop away (as it should), however turning the screw outwards to allow a richer mixture had no effect - ergo, the pilot jet is too small.
Fortunately, removing the tank, throttle cable and throttle position sensor allowed enough space to tip the carburettor over and remove the fuel bowl with the carburettor still on the bike. I removed and measured the pilot jet which turned out to be a No. 15 (15 thousandths of an inch - measured in physical hole size, not fuel flow rate). As I did not have a larger pilot jet to try, I elected to enlarge the existing jet with a 16 thou jet drill making the jet a No. 16.
Upon reassembly the idle mixture screw is now responsive in both directions and careful adjustment has produced a clean and stable idle.